The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
(1) Field of the Invention
This invention generally relates to a supercavitation ventilation control system.
More particularly, the invention relates to a supercavitation ventilation control system in which a terminal end of a cavity boundary is controlled in accordance with vehicle travel at varying speed and depth.
(2) Description of the Prior Art
Supercavitation is a means of drag reduction. Cavitation in a liquid results in gas formation. The presence of gas in the place of liquid that normally contacts an underwater body greatly reduces skin friction and thus permits higher speed travel using the same levels of propulsion thrust. FIG. 1 shows the general features of an underwater vehicle 10 having a forward end 12 and an aft end 14, the underwater vehicle 10 using supercavitation for drag reduction. The direction of travel for the vehicle 10 is shown with arrow 16. A cavitator 18 is positioned at the forward end 12 of the vehicle. The cavitator 18 is the portion of the vehicle body 10 that is in contact with the liquid 20 in which the vehicle is submersed. The motion of the cavitator 18 in the liquid 20 causes a low-pressure wake (not shown) to form aft of the cavitator 18. The pressure in the wake falls as the speed of the vehicle 10 is increased. Eventually the pressure in the wake falls sufficiently such that a vapor pressure is reached and fluid changes state from liquid to gas, forming a cavity 22 surrounding the body 10. The cavitator 18 is normally designed with a blunt forward section 18a and sharp detachment points 18b. The cavity 22 forms at the detachment points 18b. The shape of the cavitator 18 and the speed and depth of the body 10 determines the size and shape of the cavity 22. The body 10 is generally sized to utilize the cavity volume leaving space for a small clearance gap between the body 10 and the liquid 20 outside the cavity 22 designated as the cavity boundary 24. While a fore end of the cavity 22 is nearly filled with the vehicle body 10, an aft portion of the cavity 22 is nearly empty. The empty portion of the cavity 22 exhibits periodic sloshing of liquid called a re-entrant jet or a pair of vortex tubes 26 as shown.
In general, cavities formed by speed of the body alone are too small at any depth to be of practical use in drag reduction. Ventilation of the cavity is normally used to make larger cavities at a given speed or depth. In ventilated cavities, a source of high-pressure gas is introduced into the cavity. The gas causes a rapid expansion of the vaporous cavity, and the cavity continues to grow as ventilation gas enters the cavity, and the pressure in the cavity approaches the ambient depth pressure. A steady state cavity pressure is reached, as the rate of gas leakage from the cavity equals the rate of ventilation gas introduction into the cavity.
FIG. 2 shows the ability to grow a cavity by the introduction of ventilation gas. The cavitation number is the non-dimensional parameter that describes the pressure difference between the gas cavity and the ambient fluid. As the cavitation number decreases, the cavity grows in size. The Froude number is a measure of body speed and the five curves are for five constant Froude numbers increasing from curve 1 to curve 5. The ventilation coefficient is the non-dimensional parameter that describes the volumetric flow of gas into the cavity. The data shows that as ventilation gas increases, the cavitation number lowers and hence the cavity grows. At some point, gas leakage increases dramatically and ventilation flow rate increases cannot be used to expand the size of a cavity. This behavior results from the basic cavity closure in the aft of the cavity and its interaction with the liquid flow.
The body 10 must provide the volume of gas required for ventilation and cavity envelopment of the body. Thus, high gas losses caused by normal cavity closure as outlined above causes increased volumetric requirements of the body 10. This use of the body volume limits travel at certain depths and also limits the use and practicality of supercavitating bodies.
The forces on a supercavitating body are due primarily to contact of the body with wetted flow. Normally this contact is at the cavitator, control fins and the aft section of the body, which planes on the cavity interface. The control of the supercavitating body is not optimal as a result of the fluctuating cavity behavior and the structure of the normal cavity closure.
The following patents, for example, disclose cavitating structures, but do not disclose an apparatus to modify and thereby control the cavity boundary generated by a cavitator as does the present invention.
U.S. Pat. No. 3,016,865 to Eichenberger;
U.S. Pat. No. 3,875,885 to Balquet et al.;
U.S. Pat. No. 3,205,846 to Lang;
U.S. Pat. No. 5,955,698 to Harkins et al.; and
U.S. Pat. No. 6,167,829 to Lang.
Specifically, Eichenberger discloses a method and apparatus for reducing the drag of bodies or vehicles such as a torpedo or a submarine or the like submerged in a liquid such as water. More particularly, the invention relates to a method and apparatus for providing a reduction of such drag by stabilization of a laminar water boundary layer by a gas film introduced between the body and the surrounding liquid whereby the stabilization of the laminar water boundary layer also results in the stabilization of the water-gas interface.
The patent to Lang ""846 discloses a torpedo body form and gas layer control. The underwater craft includes an elongated hull having generally rounded transverse sections there along. An annular gas cavity is generated adjacent to the hull and means are provided for communicating the cavity rearward from a predetermined circumferential cavity generation locus of the hull disposed near the nose of the craft to a predetermined circumferential cavity closure and rewet locus of the hull disposed near the tail of the craft. A gas is selectively and varyingly introduced into the cavity for maintaining a predetermined communication between the loci. Means are provided for measuring the thickness of the annular cavity, the means adapted to introduce a variable quantity of gas into to the cavity. In response to the determined thickness, the quantity of gas introduced into the cavity is controlled in an inverse relationship to the cavity thickness.
Balquet et al. discloses an air injection propulsion system for marine vessels including a primary gas injector for creating an axial gas flow beneath the vessel""s hull, a primary aerator located beneath the vessel""s hull for generating an aerated flow of water, and a secondary aerator, for further refining the aerated flow, includes a deflecting surface to provide the main propulsive effect. The primary aerator comprises a contoured surface positioned transversely to the gas flow, which, in one embodiment, has located therein a series of slots with their axes parallel to the gas flow. Axial and transverse aeration of the water flow adjacent the gas flow are generated simultaneously by the primary aerator from the same axial gas flow. The primary aerator further comprises a deflecting foil spaced from and positioned opposite to the contoured surface which complements both types of aeration generated by the contoured surface. The secondary aerator comprises one or more gas injectors spaced transversely across the inclined rear surface of the vessel""s hull and one or more contoured surface diluting foils located rearward of the primary aerator and positioned transversely across the aerated flow from the primary aerator.
Harkins et al. discloses a supercavitating water-entry projectile having empennage on the aft end providing both aerodynamic and hydrodynamic stability and a supercavitation nose section is provided. A representative projectile is a subcaliber munition adapted for use in a 25 mm weapon using a sabot currently in use with the M919 round. The projectile has circumferential grooves around its center section to match these sabots. A key feature in the invention is the size and shape of the nose section. The projectile has a novel high strength extended blunt nose section followed by a truncated conical section which angles towards the body of the projectile in the range of five degrees. During underwater trajectory, the entire projectile in contained within the cavitation bubble formed by the blunt nose tip. The projectile""s aft empennage, which provides both aerodynamic and hydrodynamic stability, fits within the bore of the weapon.
The patent to Lang ""829 discloses gas filled cavities that reduce drag on the underwater surfaces of marine vehicles. Hydrofoil, struts, boat and ship hulls, pontoons, underwater bodies, fins, rudders, fairings, protuberances, submarine sails and propulsors are underwater surfaces that may be covered by the gas-filled cavities to reduce drag on them. The gas-filled cavities are to be used on underwater surfaces of marine vehicles, such as hydrofoil craft, monohulls, catamarans, small waterplane area twin hull craft, surface-effect ships and wing-in-ground effect vehicles. Each gas-filled cavity is formed by ejecting air near the end of each nosepiece. Air is ejected at a speed and direction close to that of the water at the local cavity wall. The cavity is formed behind the nosepiece. The nosepiece is adapted to control the shape of the cavity. Cavity length is also controlled through controlling air ejection rates, and through the use of a tailpiece to close the cavity within a limited region near the front of the tailpiece.
It should be understood that the present invention would in fact enhance the functionality of the above patents by providing a supercavitation ventilation control system having a cavity control ring and a stop ring, each slidably mounted on the underwater vehicle for selectively adjusting a cavity size surrounding the vehicle body and a termination point of the cavity.
Therefore, it is an object of this invention to provide a ventilation and control system for a supercavitating vehicle.
Another object of this invention is to provide a ventilation and control system for a supercavitating vehicle in which ventilation gas loss is controlled at any vehicle operating speed and/or depth condition.
Still another object of this invention is to provide a ventilation and control system for a supercavitating vehicle effective during maneuvering of the vehicle.
A still further object of the invention is to provide a ventilation and control system for a supercavitating vehicle in which ventilation control is achieved in conjunction with vehicle maneuvering systems.
Yet another object of this invention is to provide a ventilation and control system for a supercavitating vehicle in which the dimensions of the cavity are actively controlled.
In accordance with one aspect of this invention, there is provided a supercavitation ventilation control system including a vehicle body having a fore end and an aft end. A cavitator is joined to the fore end of the vehicle body, the cavitator generating a gas cavity around the vehicle body. A cavity control ring is slidably positioned at the aft end of the vehicle body, the gas cavity control ring selectively adjusting a terminal end of the cavity formed by the cavitator. A stop ring is adjustably positioned on the vehicle body forward of the cavity control ring for managing a reentrant jet generated by the cavity control ring. Each of the stop ring and cavity control ring are moveable by separate actuators and a single control system.